Field of the Invention
The present invention relates to methods and apparatus for metrology usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques. Methods of measuring critical dimension (line width) are described, as a particular application of such metrology. Methods of measuring asymmetry-related parameters such as overlay are also described.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer).
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes (SEM), which are often used to measure critical dimension (CD). Other specialized tools are used to measure parameters related to asymmetry. One of these parameters is overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis. Compared with SEM techniques, optical scatterometers can be used with much higher throughput, on a large proportion or even all of the product units.
The targets used by conventional scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In order to reduce the size of the targets, e.g., to 10 μm by 10 μm or less, e.g., so they can be positioned in amongst product features, rather than in the scribe lane, so-called “small target” metrology has been proposed, in which the grating is made smaller than the measurement spot (i.e., the grating is overfilled). These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Typically small targets are used for measurement of overlay and other performance parameters that can be derived from measurements of asymmetry in the grating structure. By placing the target in among the product features (“in-die target”), it is hoped to increase accuracy of measurement. The improved accuracy is expected for example because the in-die target is affected by process variations in a more similar way to the product features, and less interpolation may be needed to determine the effect of a process variation at the actual feature site. These optical measurements of overlay targets have been very successful in improving overlay performance in mass production. So-called dark-field imaging has been used for this purpose. Examples of dark field imaging metrology can be found in international patent applications US20100328655A1 and US2011069292A1 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and US2015138523. Similar small target techniques for focus performance and dose performance have been implemented also. The content of all these prior application is incorporated herein by reference.
As technology develops, however, performance specifications become ever tighter. Moreover, small target techniques have not been developed for measurement of other parameters such as line width or critical dimension (CD). A further limitation of current methods is that they are made with optical wavelengths, much greater than the typical dimensions of real product features. A particular parameter of interest is linewidth (CD). CD metrology suffers from low accuracy, cross-talk between parameters of interest and also between parameters of interest and other hidden parameters (process robustness). As the microscopic structures shrink and become more and more complex in geometry (going to 3-D structures, for example), known techniques of CD metrology struggle to provide accuracy, precision, and speed. Another parameter of interest is overlay.
An attractive option for improving the measurement of smaller features is to use radiation with wavelengths shorter than the wavelengths used in the conventional scatterometers. Radiation can be used in wavebands such as those known as ultraviolet (UV), deep ultraviolet (DUV) radiation, vacuum UV (VUV), extreme UV (EUV), soft x-ray (SXR) and x-ray. These wavebands are of course simply named regions of a continuous electromagnetic spectrum, rather than having any hard physical definition. The bands can be defined differently by different workers, and may overlap one another.
Reflectometry techniques using X-rays (GI-XRS) and extreme ultraviolet (EUV) radiation at grazing incidence are known for measuring properties of films and stacks of layers on a substrate. Within the general field of reflectometry, goniometric and/or spectroscopic techniques can be applied. In goniometry, the variation of a reflected beam with different incidence angles is measured. Spectroscopic reflectometry, on the other hand, measures the spectrum of wavelengths reflected at a given angle (using broadband radiation). For example, EUV reflectometry has been used for inspection of mask blanks, prior to manufacture of reticles (patterning devices) for use in EUV lithography. Work on these techniques has been described for example by S Danylyuk et al in “Multi-angle spectroscopic EUV reflectometry for analysis of thin films and interfaces”, Phys. Status Solidi C 12, 3, pp.318-322 (2015). However, such measurements are different from the measurement of CD in a periodic structure. Moreover, particularly in view of the very shallow grazing incidence angles involved, none of these known techniques is suitable for metrology on small targets such as an in-die grating.
In pending international patent application PCT/EP2016/056254, not published at the present priority date, it is proposed to measure properties such as CD and overlay of target structures using EUV radiation, that is radiation in the wavelength range from about 1 nm to about 100 nm. Spectroscopic reflectometry is performed using radiation scattered at zero and/or higher diffraction orders. A smaller spot size is achieved than in T-SAXS or GI-SAXS methods, using a higher grazing angle of incidence than can be used at x-ray wavelengths. Diffraction signals are further strengthened by the use of a conical mount between an EUV optical system and the substrate. This allows a non-zero azimuthal angle of incidence relative to a direction of periodicity of the target structure. The contents of the prior application are hereby incorporated by reference in the present disclosure.
Such techniques using radiation in the extreme ultraviolet (EUV) waveband offer particular advantages for metrology of CD, overlay and other properties of small metrology targets. Conveniently these small metrology targets may again have the form of periodic structures. Compared with the optical scatterometry commonly practiced, EUV rays will not be strongly influenced by underlying features, and modeling of the periodic structure itself can be more accurate as a result. In order to obtain sufficient information for CD metrology, spectral properties across a range of EUV wavelengths can be measured. On the other hand, making measurements of a target structure using radiation in different wavelength ranges and/or using different properties of the radiation can be beneficial. For example, when measurements using radiation in different wavebands are combined in a reconstruction or similar method, accuracy in the calculated measurements of a property of interest can be improved. When using radiation in a single waveband, cross-correlation between different properties of the material and/or geometry of the structure can lead to error and/or uncertainty in a calculated property of interest. Using information from additional wavebands, some of these errors and uncertainties can be resolved. Of course, measurements in different wavelength ranges can also be used independently to measure two different properties.
In the mentioned international patent application, a form of hybrid metrology is proposed in which larger targets with product-like structures are measured using the EUV radiation, while smaller in-die targets are measured using an angle-resolved scatterometer working in a more conventional optical waveband. The results of the EUV measurements on a few substrates are used to calibrate the optical measurements in high-volume manufacture. In a European patent application 15202273.7, also not published at the present priority date, metrology apparatus based on EUV spectroscopic reflectometry and an angle-resolved scatterometer working in (for example) the optical waveband are combined in a single apparatus. The contents of the European patent application are incorporated herein by reference. Although in these prior applications the apparatus required for making measurements at different wavelength ranges is housed together and shares certain common infrastructure, the different measurements are nevertheless are made using separate sources and optical systems within the same apparatus.